Range-gated digital amti system

ABSTRACT

A pulsed energy system for airborne use and having range-gated digital means for doppler processing of moving-target signals. A digitizer generator provides a first pulse train, indicative of the video output of the system receiver, as an up-count input to an up-down counter. An adjustable pulse train, (corresponding to the function Vacos theta ) is applied as a down-count input to the up-down counter. A synthetic video, or display, signal is gated-on in response to an overflow condition of the up-down counter.

United States Patent McFarland Dec. 11, 1973 [54] RANGE-GATED DIGITALAMT! SYSTEM 3.737900 6/1973 Vehrs 343/5 DP X [75] Inventor: Wilmer H.McFarland, Fullerton,

m Primary E.\'aminerT. H. Tubbesing I Attorney-William R. Lane. L. LeeHumphries and [73] Assignee: North American Rockwell Rolf pinsCorporation, El Segundo, Calif.

[22] Filed: Feb. 27, 1967 57 ABSTRACT A pulsed energy system forairborne use and having range-gated digital means for doppler processingof moving-target signals. A digitizer generator provides a 52 U. Cl. 3437.7 328 151, 343 5 DP 1 S first pulse train, indicative of the videooutput of the [51] Int. Cl. G015 9/42 I 58 Field of Search 343/5 DP,7.3, 7.7; System mew, as up'down 328/151 165 counter. An adjustablepulse train, (corresponding to the function V cos 0) is applied as adown-count input [56] References Cited to the up-down counter. Asynthetic video, or display, signal is gated-on in response to anoverflow condition lguTED STATES PIATENTS of the up-down counter.3,720,942 l973 Wilmot et a 3,725,923 4/1973 Bosc et al. 343/7.7 23Claims, 10 Drawing Figures '1 A RANGE 1 DIGITAL AMTI 1 824 "u IPROCESSOR ,Nau l V cos5: (b l I RANGE i DIGITAL AMTI 1 I PROCESSOR -M- ui ur I GATE I o M V0 case: \l3b 3 D I u l- '1 I VEHICLE I r BORNE IAIIII l an? I are I SYSTEM T I q @l. l5 I .I I I I ANTENNA, {6 L jPICK-OFF 7 FUNCTION l GENERATOR ig c sl i i. I 059 HAVING s h we ISENSOR MULTIFLIER I7 I FREQUENCY I s i rgateige a: I. 8 Q ,E f

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WILME'RIf MC'FARLAQQ- CROSS-REFERENCE TO RELATED APPLICATIONS 1.Application Ser. No. 391,073, filed Aug. 18,1964, by F. J. Dynan, etal., for AMTl Radar System, now U.S. Pat. No. 3,408,647.

2. Application Ser. No. 552,556, filed May, 24,1966, by W. R. Fried, etal, for Platform Motion Compensation of a Coherent MTl System, now U.S.Pat. No. 3,341,847.

BACKGROUND OF THE INVENTION The utility of a non-coherent dopplerprocessor in a pulsed energy system, such as an airborne radar systemfor the detection of moving targets, has been described in a copendingapplication Ser. No. 391,073 for an AMTl Radar System, filed Aug. 18,1964, by F. Dynan et al., assignors to North American Aviation, Inc.,assignee of the subject invention. The display of a substantiallyclutter-free moving target by means of employing such non-coherentdoppler processor, relies on the presence of a substantial cluttercontent in the received signals, which gives rise to aclutter-referenced moving target signal which may be spectrallydistinguished from the d-c, or low-frequency, energy of the detectedclutter content. For such purpose a high-pass, analog doppler filter isemployed, having an upper corner frequency (or cut-off frequency) notexceeding one-half the pulse repetition interval of the pulsed energysystem employing such device.

Such prior-art arrangement has several inherent disadvantages. First,the doppler processor requires a substantial storage medium for storinga plurality of successive range trace signals (several hundred or more)for creating a data matrix of range bin versus pulsed interval, in orderthat the data in a given range bin may be sequentially scanned (in thesame sequence as the occurrence of the pulsed intervals in which thedata were recorded), in order to reconstruct the clutterreferencedmovingtarget spectra. Secondly, the fixed bandpass of the analog dopplerfilter means may respond to the clutter spectra, as the bandwidth of theclutter spectra spreads with increases in platform velocity and withchanges in look-angle (in a scanning system), while a moving target maynot be detected,

eventhough spectrally distinguishable from the clutter and below thefixed bandpass of the high-pass analog doppler filter. The blind-speedeffects imposed by the use of fixed bandpass analog filters may berelieved to some extent by the use of filter chains comprising severallyswitchable narrow-bandpass filters for covering a' doppler bandpass ofinterest, as described in the above noted copending application Ser. No.391,073. However, such switching of discrete portions of the bandwidthmerely tends to reduce, rather than avoid,

such effects. Also, the attempt to use narrow band analog filtersinvolves the problem of filter ringing or undamped responses which tendto persist some time after the application of the input giving rise tosuch response, thereby worsening the range resolution of the displaysignal. Further, use of a filter chain of severally switchable analogfilters involves the necessity of 1 switching logic, without yetproviding a continuously adjustable lower corner-frequency.

SUMMARY OF THE lNVENTlON By means of the digital doppler filter conceptof the subject invention, the above-noted shortcomings and disadvantagesof prior art "analog AMTI processors are avoided.

In a preferred embodiment of the invention there is provided aperiodically-sampled video signalling system such as an airborne movingtarget indicating type 0 of range-gated pulsed energy ranging system.Also provided is a bandpass-limited digital filter, useful forapplication as a doppler processor, and comprising a digitizerresponsive to a sampled video signal for providing a pulse trainindicative of a non-zero frequency spectral content of the sampledvideo. Such digitizer may also include threshold means for limiting theoutput response to such spectral component only where above apreselected threshold amplitude. Periodic pulse generating meansprovides an output having a periodicity corresponding to a selectedlower limit bandpass frequency. An output up-down counter having anupcount input responsive to the output of the digitizer and having adown-count input responsive to the output of the periodic pulsegenerating means provides an overflow condition indicative of thepresence of a video envelope spectral component in the frequency regionabove the lower limit frequency, which overflow condition may beutilized in an AMTI system for generating a synthetic video AMTI signalfor display purposes.

By means of the above described digital arrangement, doppler filteringmay be accomplished in an AMTI system without the necessity of storinghundreds of successive range trace signals. Only one range-gatedrange-trace signal is stored in the digitizer for comparison purposes inthe processing of the next subsequently occurring range trace signal.Therefore, the large storage capacity of previous range-gated datamatrix systems is neither required nor employed. Also, because of thedigital counter overflow technique employed for indicating the presenceof a video envelope spectral component above a selected lower limitfrequency, the resolution problems associated with filter ringing(whether an underdamped analog filter or a digital differential analyzerequivalent is employed) are avoided. Further, the use of a down-countpulse train of adjust able periodicity for establishing the lower limitfrequency of the bandpass response of the output down counter provideshighly effective means of selectively 'varying the filter bandpass.

tem look-angle or increases in platform velocity; or

may be decreased as the clutter spectrum is narrowed, in order to detectmoving targets below an initial lower limit bandpass frequency andoutside the narrowed clutter spectra. Further, such convenient means forvarying the doppler filter bandpass obviates the need for discreteswitching of the filters of a filter chain.

Moreover, where thresholding is included in the digitizer, the presenceof a moving target may be detected amid the skirt of the cluttersectrumlwith less likelihood of a pulse alarm occurring from a systemresponse to such skirt of the clutter spectrum, itself, or to noise.

Accordingly, it is a broad object of the invention to provide animproved AMTl processor.

range bin-of a receiver range trace signal.

These and other objects of the invention will become more readilyapparent from the following description taken in conjunction with theaccompanying drawings, in which:

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a block diagram of a systemembodying the inventive concept;

FIG. 2 is a block diagram of an alternate arrangement of the range-gatesignalling means of FIG. 1;

FIG. 3 is a block diagram in further detail ofa portion of the digitalAMTI processor of FIG. 1;

FIG. 4 is a family of time histories illustrating the response of thedevice of FIG. 3 to a clutter-referenced clutter return;

FIG. 5 is a family of time histories illustrating the response of thedevice of FIG. 3 to a clutter-referenced maximum speed moving targetsignal;

FIG. 6 is a family of time histories illustrating the response of thedevice of FIG. 3 to a clutter-referenced intermediate-velocity movingtarget signal;

7 FIG. 7 is a spectral diagram of a clutter referenced moving targetvideo signal, corresponding to an output of system 10 of FIG. 1;

FIG. 8 is a schematic arrangement in block diagram of an exemplaryup-down counter, corresponding to block element 21 of FIG. 3;

FIG. 9 is a schematic arrangement of an exemplary counter correspondingto block elements 26, 27, 28 and 29 of FIG. 3; and

FIG. 10 is a schematic arrangement of an exemplary counter correspondingto block element 34 of FIG. 3.

In the figures, like referenced elements refer to like parts.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to FIG. 1, thereis illustrated in block diagram form a system embodying the inventiveconcept. There'is provided a pulsed energy ranging system 10, such as aradar system adapted for airborne use and providing a clutter-referencedvideo output. Such means for providing a clutter-referenced video outputmay comprise a non-coherent system, as disclosed more fully inapplication Ser. No. 391,073 filed Aug. 18, 1964, by F. .l. Dynan etal.,,assignors to NorthAme rican Aviation, Inc., assigneefof thejsubjectinvention; alternatively, such means may comprise a coherent system, asdisclosed more fully' in' application Se'r. No. 552,556 filed May 24,1966, by W. R. Fried et al., now u.s. Pat. 3,341,847 issued Sept. 12,1967. There is also provided range gating means 11 responsive to thesystem trigger andthe'video output of the radar system for providing aplurality of range-gated-outpu t's, each outpu t're'presentingrange-gate sampling off a successive o'f delayed outputs, eachsuccessively delayed relative to a preceding one and corresponding to asuccessive one of a plurality of range-time intervals or range bins tobe sampled, for operation of range gates 11 11,, and 11 Hence, it is tobe appreciated that range gating means 11 provides a sampled videooutput on each of the output lines thereof, the sample on a successiveone of the output lines corresponding to a successive range bin of asampled video range trace signal. Although only three range-gatedoutputs are shown, it is understood that means may be provided forobtaining as many range-gated outputs as desired.

Also, although the source 12 of range gating signals has been shown as atapped delay line in the arrangement of FIG. 1, it is clear that sucharrangement is exemplary only and that other means may be employed, suchas, for example, digital shift register means, one arrangement of whichis shown in FIG. 2. In such alternate range-gate signal source, there isprovided a gated oscillator or clock generator 50 responsively coupledto the system trigger of radar system 10 (of FIG. 1) for providing aperiodic pulsed output having a periodicity corresponding to the desiredrange time interval or increment between successive range bins, whichserves as a shift pulse source. The shift register itself may becomprised of a series or chain of flip-flops, the I state output of eachbeing connected to the reset or 0 input of the succeeding flip-flop ofthe series, and the 0 state output being connected to the set of 1 inputof the succeeding flip-flop, each input of each flip-flop stage beinggated by shift pulse source 50. In this way, the output state of eachflip-flop provides a corresponding enabling input to the next flip-flop.The next flip-flop, however, cannot respond to the enabling input toassume the state of the preceding flip-flop until the application of ashift pulse during such enabling state. Because of a preselected delayprovided by the combined response time of each gate and an associatedflip-flop, in responding to the shift pulse, the state change of oneflip-flop stage (in response to the coincidence of a changed enablestate and a shift pulse) and does not result in concomitant statechanges in succeeding ones of the chain of flip-flops. Accordingly, astate-change to the 1 state in flip-flop FF-l (at a time correspondingto a first range-bin of interest) while producing a change in the erable statEZinput-to the second flip-. flop Qs'tage, does not result in astate change to the 1 state of then'ext flip-flop FF-2 until theoccurrence of the next shift t-pulse (from source 50) at time t(corresponding to a second range bin of interest. At the same time (orwithin a. gate-delay response interval thereafter), such shift pulsealso serves to reset the first and enabling t'lips flops FF-() and FF-l.This same sequence'occur's in each o f successive flip-flop stages inresponse to a successived'iie of the series of shift pulses from pulsesource 50. In this way, a 1 state or'conditionis sequentially shifted bya first flip-flop to a next one in a series of flip-flops, and a rangegating signal is provided at each of the gate control output lines atthat one ofa se- 5 1161166 of intervals corresponding to a mutuallyexclusive one of a series of range bins to be sampled, lll accordancewith the following table:

FFI FF-Z FF 1, 0 0 I, l 0 0 r 0 l O 1, 0 0 r 1,, 0 0 O r l 0 0 Theconstruction and arrangement of shift registers are generallyunderstood, as indicated at page 246247 of Logical Design of DigitalComputers" by Montgomery Phister, Jr., published by John Wiley and Sons,Inc., 1958, and at page 20, 21 of The Digital Logic Handbook, 1966-67Edition, published by Digital Equipment Corporation of Maynard, Mass,and in U.S. Pat. No. 3,295,107 issued Dec. 17, 1966, to R. E. Stalcup.

The range gated video signal at each of the outputs of range-gatingmeans 11 is fed to a mutually exclusive one of a plurality of digitalAMTI processors 13a, 13b and 13m for digital doppler processing of thesampled l or ranged video signal, and generation of a synthetic videosignal in synchronism with a mutually exclusive one of the range-gatingcontrol signals and indicative of a moving target within the range bincorresponding to such range gating control signal. Accordingly, bycombining the synthetic video outputs of doppler processors 13a, 13b andl3n by means of signal combining means 14, a doppler-processed videorange trace signal is provided.

Each of like digital processors 13a, 13b and 13m is comprised of adigitizer 15 coupled to the output of an associated one of range gates11a, 11b, lln; and a digital filter 16 coupled to the output ofdigitizer l and having an adaptive lower bandpass frequency. Digitizerprovides the triple functions of digitzing the sampled video data, delayline cancelling the zero frequency or d-c component of the videoenvelope of the digitized video sample, and thresholding of the non-zerofrequency component of the digitized video envelope. Digitizer 15 mayalso serve the additional function of upper band pass limiting the videoenvelope. Digital filter 16 serves the triple functions of furtherbandpass limiting the threshold digitized signal, automaticallyadjusting the bandpass in response to an arbitrary function, andgenerating a synthetic video output pulse in synchronism with theperiodic range time interval associated with the range gated video inputto the digital processor. By reason of these latter two functions,filter 16 is further responsively coupled to a multiplier 17 and torange gate control means 12.

The construction and arrangement of digital processor l3n is shown infurther detail in FIG. 3. Each of digital processors 13a, 13b and l3nare similarly constructed and arranged, and therefore, a furtherdescription. in detail of only processor 13n will suffice.

Referring to FIG. 3, there is illustrated in block diagram form aschematic arrangement of doppler processor 13!: of FIG. 1. There isprovided a closed loop ana log-to-digital (A/D) converter 18 and atrigger generator 19, comprising digitizer 15 of FIG. 1. A/D converter18 is comprised ofa pulse train generator or burst oscillator 20responsive to a preselected range bin gating signal output (from rangegate control means 12 of FIG. 1) for providing a pulse train input intoan updown counter 21 having an up/down control input responsivelycoupled to the output of an-analog differen- 6 ier 22. Differentialamplifier 22 has a first tial amplif l puires orln ii sample provided inresponse to that gating control signal employed as an input to pulsetrain generator 20,

and has a second input responsive to the analog of the digital valuestored in counter 21. The range gate lln, interposed betweendifferential amplifier 22 and the receiver video output of radar system10 (of FIG. 1) may be comprised of a gated box car detector in order toboth sample and hold the (range-gated) video signal. In this way, thesampled video signal input is temporarily stored long enough for theclosed loop A/D converter 18 to respond. The analog of the digital valuestored by counter 21 is provided by a digital-to-analog (D/A) converter23.

In normal operation of closed-loop A/D converter 18, a pulse train ofapreselected number of pulses (say, eight pulses) is applied at a clockinput of counter 21 in response to a range gate trigger applied to gatedburst oscillator 20, thereby causing counter 21 to count or integratesuch applied pulse input, combining such count with the count storedtherein from a range-gated input occurring during a preceding pulserepetition interval of pulsed radar system 10 (of FIG. 1). The sense ofthe difference between the sampled video input (form boxcar detector 11n) and the output of D/A converter 23 is indicated at the output ofdifferential amplifier 22, and employed to control the counting actionof counter 21 in such a sense or direction as to tend to reduce themagnitude of the difference between the sampled video and the D/Aconverter output to zero, as is well understood in the art.

The up-down control signal from differential amplifier 22 is alsoapplied as an input to trigger generator 19 for the generation of atrigger by an appropriate one of monostable multivibrators 24 and 25 inresponse to each change of sense of the output of differentialsignalling means 22. Such change of sense will not occur in response toa sampled d-c video envelope at a gated range or range bin of interest,because the difference between a stored video amplitude (at the outputof D/A converter 23) and the next range-gated video sample (within suchrange bin) would be zero, resulting in a null at the output ofdifferential amplifier 22. Therefore, no trigger output would begenerated in response to such up-down control signal. If, however, thesampled video input to closed-loop A/D converter 18 has a non-zerofrequency (or time-varying) component, such component will result in achange in the sense of the up-down control output of amplifier 22, eachsense change being employed by trigger generator 19 to generate aunipolar pulse output. Where such time-varying video componentrepresents a single frequency (or spectral line) then such pulses willbe generated by trigger 19 at a periodicity corresponding to twice thesingle frequency. In other words, the sense of the counter controlsignal output from amplifier 22 reverses twice each cycle of a singlefrequency time varying component of a video envelope resulting in apulse output from generator 19 every half cycle. Hence, A/D converterand trigger 19 cooperate as a pulse canceler to provide a trigger outputindicative of only a non-zero frequency component of a sampled videoenvelope.

Where it is desired to limit or prevent the response of triggergenerator 19 to low-level noise or spurious inputs, threshold means maybe incorporated in trigger 19 to prevent response thereof to signalsbelow a preselected threshold (corresponding to the residue or skirt ofthe clutter spectra) or of less than a minimum dura tion in time(corresponding to system noise). Such signal limiting means in thearrangement of FIG. 3 comprises a four-bit up-down counter 26 having anupcount output 31 coupled to multivibrator 24 by a first threshold.comparator 27 and further having a downcount output 32 coupled tomultivibrator 25 by a second threshold comparator 28, as well as havingan updown controlinput coupled to the up-down signal output of amplifier22.

An output of each of comparators 27-and 28 is fed back to an input of anAND gate 29 which gate is further responsively coupled to the output ofpulse train generator 20. Whenever the digitized video envelope goesthrough a point of infliction (as indicated by a change of state orsense reversal of the control signal output from amplifier 22 of A/Dconverter 18 of FIG. 1), then counter 26 is reset and commences to countthe decrease-or change in signal magnitude from the value existing atsuch point of inflection. A change or output count greater than thepreselected threshold setting of an appropriate one of comparators 27and 28 produces a change in state of the output of such comparator,which is fed to an input of an associated one of multivibrators 24 and25, resulting in a pulsed output on line 33. Such pulsed output willthus occur twice each cycle for a single (non-zero) frequency videosignal, and hence would be periodic in occurrence. Where the amplitudeof the (non-zero frequency) single frequency video envelope is below thepreselected threshold, no such periodic output will occur on line 33.

A thresholded state in the output of one of comparators 27 and 28, dueto the input thereto exceeding the preselected threshold, is employed asan inhibit input to logic gate 29. In other words, for an input to arespective one of comparators 27 and 28 below the preselected thresholdlevel, the output state of such comparator comprises an enable input tothe input logic of counter 26.

Although trigger generator 19 has been described as threshold means forgenerating two pulses per cycle or one trigger pulse per half cycle of a(non-zero frequency) sample frequency video envelope, the response rateof trigger generator 19 is limited to nonzero frequencies of less thanthe system PRF of the pulsed transmitter (of system in FIG. 1). In otherwords, trigger generator 19 tends to cooperate as an upper bandpasslimiter. Such limited response is obtained by limiting the speed of thepulse train provided by pulse train generator in cooperation with theselection of the threshold of comparators 27 and 28, whereby the counter26 cannot respond rapidly enough to higher frequency thresholdamplitudes to provide a counter output at least as great as thepreselected threshold. In this way, the response of trigger generator 19is limited to no more than one pulse output per system pulse repetitioninterval, corresponding to one pulse per half cycle for a single videofrequency not exceeding the system PRF.

The upper bandpass limited output 33 of trigger generator 19 is fed toan up-count input of an output counter 34 of a synthetic video signalgenerator 40, a down-count input of output counter 34 being responsivelycoupled by line 36 to a source 35 of a lower limit frequency. Where thetrigger generator output pulse train rate at the up-count input isgreater than the down-count pulse train rate of source 35, an overflowcondition will ultimately develope at an output of counter 34. Thelength of time or number of system pulse repetition intervals requiredfor such overflow condition to develop depends upon the amount of thefrequency difference between the up-count and downcount pulse trains andthe most significant bit or preselected capacity of the counter 34.

The overflow condition of output counter 34 may be gated by the rangebin gating signal by means of a coincidence gate 37, to generate asynthetic video output signal at a range time corresponding to therange-bin sampled by the range-gating signal. Where, of course, theperiodic output of trigger generator 19 corresponds to a vdeo envelopespectral component below the frequency of source 35, then no overflowcondition occurs at the output of counter 34, and no synthetic video isgenerated at the output of gate 37.,Hence, it is to be appreciated thatsynthetic video signal generator 40 cooperates with the upper bandpasslimited output of trig;

ger generator 19 and the single frequency output of oscillator 35 as abandpass limited filter, the upper cut-off or limit frequency of whichcorresponds to (PRF/2) and the lower cut-off or limit frequency of whichcorresponds to the periodicity of oscillator 35.

The cooperation of elements 18, 19, 35 and 40 as a digitizer and digitaldoppler filter may be further appreciated from a consideration of FIGS.4, 5 and 6.

Referring to FIG. 4, there is illustrated a family of time histories ofthe arrangement of FIG. 3 to a d-c, or zero-frequency, video envelope asa function of periodic (radar) time. The notation t refers to theoccurrence of the system trigger of the pulsed energy system 10 (of FIG.1), while the notation, t, refers to an exemplary range-gated time orrange bin of interest. Curves 41, 42, 43, 44 and 45 respectivelyrepresent the respective outputs of gated boxcar detector 11n, clockgenerator 20, up-down control amplifier 22, D/A converter 23, andtrigger generator 19 of FIG. 3. Curves 46, 47 and 48 respectivelyrepresent the respective outputs of oscillator 35, output counter 34 andcoincidence gate 37. It is to be noted that upon the digitizerintegrating that portion of clock pulse train 42 occurring within theup-count control interval of the up control interval of the up controlsignal 43, no further change occurs in the D/A converter output, becauseno further changes occur in the video envelope of the sampled videosignal 41. Upon the counter 26 of trigger generator 19 (in FIG. 3)counting up to, say, a preselected threshold of four clock pulses of theinitial input of clock train 42, an initial pulse output 45 occurs,indicative of the threshold condition being exceeded. However, becauseno further state change occurs in the video envelope, no further triggerpulses occur in the trigger generator output 45 in FIG. 4. Such lack ofa further and periodic output 45 (for trigger generator 19 of FIG. 3)corresponds to the absence of a non-zero frequency component of thethresholded video envelope.

Accordingly, the periodic output 46 (applied to the down-count input ofcounter 34 of FIG. 3) prevents an overflow condition from occurring atthe output 47 (in FIG. 4) of counter 34. Therefore, no synthetic videosignal results in the output 48 (in FIG. 4) of the synthetic videosignalling means 40 (of FIG. 3).

Where, however, the video envelope represented by the sampled videoinput has a time-varying, or non-zero frequency, component correspondingto (PRF/2), as

shown by curve 41 in FIG. 5, then the up-down control signal 43 changesstate in synchronism with such component, resulting in a correspondingtime-variation in the output 44 of the D/A converter. The thresholdtrigger generator 19 responds to provide an output pulse 45 each timethe time-varying video component goes through a point of inflection(i.e., a change in the sense of the rate of change) with an associatedamplitude change at least equal to the preselected threshold. Suchoutput 45 of trigger generator 19 occurs each half cycle of the(non-Zero) single frequency (PRF/Z) video component utilized in theexample of FIG. 5, resulting in an up-count pulse every system pulserepetition interval of the pulsed energy system (of FIG. I). The timeintegral of the difference between the up-count pulse train 45 (at arate (PRF/2) and a down count pulse train 46 (at a rate less than(PRF/Z), eventually results in an overflow condition in the output 47(in FIG. 5) of the output counter 34 (of FIG. 3). Such overflow may bepreselected at any level to assure a noise-free output reliablyindicative ofa moving target. In FIG. 5, at least a single pulse countdifference or level has been employed which, in coincidence with therange'gate signal occurrence (at periodic time t,,) at coincidence gate37 (of FIG. 3), results in the generation of a synthetic video pulse 48at range time 1,, (in FIG. 5).

Where the (non-zero) single frequency component of the video envelope isless than one-half of the down count pulse train, of course no overflowcondition will occur in the output 34 and no synthetic video pulse willresult at the output of gate 37.

As shown in FIG. 6, where a (non-zero) singlefrequency component of thevideo envelope is less than one-half the system PRF, but yet greaterthan one half the frequency of the down count pulse train, the buildupto an overflow condition in the output 47 of the output counter incoincidence with the range gate time t,, requires more system pulserepetition intervals in which to generate the first one of the syntheticvideo pulses 48.

Such synthetic video condition of FIGS. 5 and 6 corresponds to thespectral situation illustrated by cusp 51 in FIG. 7, which is seen tolie within the filter bandwidth of curve 52 (between w and (PRF/2). Thelow frequency clutter cusp 53, seen to be folded about zero or d-c inFIG. 7, is in general spread spectrally about zero, rather than beinglimited to a single spectral line at zero frequency, as is wellunderstood in the AMTI system art. Although the clutter cusp 53 is shownas not extending above the lower bandpass limit, or cut-off, frequency mthe skirt or spectral edge of such cusp actually may have a finitesignal level extending into the filter bandpass 52, for which reason theabove described thresholding of the digitizer is to be preferred. Inthis way, the broadest practical bandpass (or lowest cut-off frequency mmay be used in order to maintain surveillance over the broadest range ofdoppler speeds for which a moving target may be detected. Where, howeverthe spectral spread of the clutter cusp increases (as shown by curve 153in FIG. 7), due to increases in the look-angle or platform velocity of autilizing AMTI pulsed energy system, then it may be preferable toincrease the filter lower-limit or cut-off frequency, as indicated by w'thus restricting the filter bandpass (curve 152 in FIG. 7).

Such variation in the video bandpass characteristics of the digitaldoppler filter of FIG. 3 may be achieved by employing a voltagecontrolled oscillator for source 35, and responsively coupling thecontrol terminal thereof to means 17 for generating a control signalindicative of the product V cos 0 of the platform velocity V,, andcosine-of the system look-angle, as shown in FIG. 1. Such means maycomprise a signal multiplier 17 having a first input responsive to asignal indicative of platform velocity and a second input responsive toa function generator providing a signal indicative of the cosine of thesystem look angle.

The arrangement and cooperation of counters 21, 26 and 34, representedin block form in FIG. 3 for convenience, are generally understood in theart. However, exemplary arrangements thereof are more fully illustratedin FIGS. 8, 9 and 10.

Referring to FIG. 8, there is illustrated in schematic form an exemplaryarrangement of counter 21 of FIG. 3, comprising a least-significant-bitflip-flop (LSB FF), most-significant-bit flip-flop (MSB FF),intermediate flip-flop stages and associate logic gating, wherebysuccessive clock pulse inputs c, to LSB FF produce alternate inputstates of such flip-flop a binary count of which is carried on bysuccessive flip-flop stages. A selected polarity stage of the up-countline inputs to the flip-flop gating logic assures that a preselectedflip-flop state is associated with a count-up" in a successive flip-flopstage, while the down-count line, connected to the up/down controlterminal by an inverter 57, provides a reversal of the flip-flop statesequence or binary count associated with a reversal of the polaritystate of an up/down control input applied to control terminal 58. Sucharrangement of basic binary counting element 21 may be employed in eachof the arrangements of counters 26 and 34 as shown in FIGS. 9 and 10.

Referring to FIG. 9, there is illustrated a portion of pulse generator19 of FIG. 3, including a counter 26 arranged substantially inaccordance with that of FIG. 8, an input gate 29, and comparators 27 and28. Comparators 27 and 28 are seen to be essentially logic gatesresponsive to the counter stage for providing a two-state outputindicative of a preselected counter stage corresponding to a thresholdcount-down occurring during a selected state of an up down control inputapplied to control terminal 58. The subsequent preselected countassociated with a given change of the control signal state and appliedto gates 27 and 28, corresponds to a change from a point of inflectionin the video envelope indicative of a non-zero frequency component in excess of the threshold level. The resultant state of a corresponding oneof gates 27 and 28 inhibits gate 29, whereby no further clock pulseinputs 0,, are applied to the counter. A subsequent and further reversalof the state of the control input at terminal 58 removes such inhibitfeedback signal as to allow the application of further clock pulseinputs c to the counter, while reversing the counting direction or sensethereof. Thus a complimentary counting state is employed with suchopposite polarity or control state of the control signal by a second oneof gates 27 and 28, to generate an inhibit signal when the videoenvelope deviation from a second point of inflection exceeds thepreselected threshold. Such means of indicating a periodic videocomponent having an amplitude in excess of a preselected thresholdconceptually differs from prior art means which determine the occurrenceof a bipolar signal level of a preselected amount above a null. Anadvantage of the disclosed technique for measuring a de- I viation froma point of inflection is the ability to respond to a selected spectralcontent including more than a single frequency, while providing anidentification of such spectral content.

The output counter 34 of FIG. 3, as disclosed more fully in FIG. 10, isseen to comprise the basic counter of FIG. 9, and pulse translatingmeans 59 for commonly employing both the up-count and down-count pulsetrains of like unipolar clock inputs 0,, to the gated counter 26, whileutilizing the up-count pulse train (on line 133) as an up-count controlsignal (of a first sense) and employing the down-count pulse train (online 136) as a down-count control signal (having a sense opposed to thatof the up-count control signal). Where, however, the normal state of thecounter is a downcount state (in the absence of an up-count controlinput on line 133), then an explicit down-count control input line 136need not be employed.

Accordingly, it is to be appreciated that there has been describeddigital means for the doppler processing of the range gated video outputof a pulsed energy system. Although the device has been described interms of the response to a sampled video envelope having only a singlefrequency component, it is to be understood that such description isexemplary only, and that the device functions satisfactorily as abandpass filter for any video envelope. Also, the filter process of thedigital means disclosed does not rely on the storage of a plurality ofsuccessive range-gated range-traced signals, but employs only thestorage of that single range trace signal immediately preceding acurrently sampled range-trace signal. Further, no digital equivalent ofan analog filter network is employed, and hence filter ringing problemsare avoided; and the lower corner frequency of the bandpass of suchfilter may be conveniently adjusted. Moreover, the use of a thresholdeddigital counting device for generation of a synthetic video reduces thelikelihood of noise and extraneous outputs in the doppler processedsynthetic video signal. Therefore, an improved digital filter has beendisclosed.

Although the digital doppler processor and associated pulse traingenerator have been described in terms of processing a single range-bin,with a plurality being required for the range-bins of a range tracesignal, it is to be understood that such device may be timesharedbetween a number of range-bins, whereby a fewer number of such devicesare required to implement the inventive concept. Such time-sharingpossibility arises due to the small portion of the pulse repetitioninterval imployed by the trigger generator (burst oscillator) togenerate the clock pulses utilized.

Although the invention has been described and illustrated in detail, itis to be clearly understood that the same is by way of illustration andexample only and is not be taken by way of limitation, the spirit andscope of this invention being limited only by the terms of the appendedclaims.

I claim:

1. In a video sampled data system, digital filter means for indicatingthe presence of a spectral component in the video envelope of the outputof said system and above a preselected lower limit frequency, andcomprising:

digitizer means responsive to the amplitude and sampled interval of thesampled video of said system for providing a pulse output indicative ofa nonzero frequency component of said video envelope; periodic pulsegenerating means having a periodicity corresponding to a selected lowerlimit bandpass frequency; and an up-down counter having an up countinput responsive to the output of said digitizer means and a down countinput responsive to the output of said pulse generating means forgenerating an over flow condition indicative of the presence of a videoenvelope spectral component in the frequency region above said lowerlimit frequency.

2. The device of claim 1 in which the periodicity of said periodic pulsegenerating means is adjustable, whereby said selected lower limitbandpass frequency is correspondingly adjusted.

3. In a video sampled data system, digital filter means for indicatingthe presence of a spectral component in the video envelope of the outputof said system and above a preselected lower limit frequency, andcomprising:

a digitizer means responsive to the amplitude and sampled interval ofthe sampled video of said system for providing a pulse output indicativeof a non-zero frequency component of said video envelope above apreselected threshold amplitude;

periodic pulse generating means having a periodicity corresponding to aselected lower limit bandpass frequency; and

an up-down counter having an up count input re- 30 sponsive to theoutput of said digitizer means and a down count input responsive to theoutput of said pulse generating means for generating an overflowcondition indicative of the presence of a video envelope spectralcomponent in the frequency region above said lower limit frequency.

4. In a periodically sampled video signalling system, a bandpass limiteddigital filter having an upper and a lower cut-off frequency andcomprising digitizer means responsive to the sampled video signal and tothe sampling periodicity of said signalling system for providing a pulsetrain indicative of a non-zero-frequency spectral content of saidsampled video signal less than one-half the frequency at which saidsampled video signal is sampled; and

synthetic video means responsive to said sampling periodicity and tosaid non-zero-frequency pulse train for providing an output pulse insynchronism with said sampling periodicity.

5. The device of claim 4 in which said synthetic video means includesmeans for limiting the lower cut-off frequency of the bandpass of saiddigital filter and comprising a reference periodic pulse generatorhaving a frequency less than one-half the sampling frequency of saidsampled signalling system and corresponding to a lower limit frequencyof the bandpass of said filter;

an up-down counter having an up-count and a downcount input, one of saidinputs being responsive to a pulse train output of said digitizer and asecond input responsive to a pulse train output of said reference pulsegenerator; and

coincidence signalling means responsive to said sampling periodicity anda preselected condition of said counter for generating an output signal.

6. The device of claim 5 in which the frequency of said referenceperiodic pulse generator is adjustable,

whereby said lower cut-off frequency of said'digital filter may becorrespondingly adjusted.

7. The device of claim 6 in which there is further included means forvarying the frequency of said reference periodic pulse generator as afunction of the velocity of a utilizing vehicle.

8. The device of claim 6 in which there is further included means forvarying the frequency of said reference periodic pulse generator as afunction of the look angle of a utilizing sensor system.

9. The device of claim 6 in which there is further included means forvarying the frequency of said reference periodic pulse generator inaccordance with the product of vehicle velocity and cosine of sensorlook angle of a utilizing sensor mounted on a moving platform.

10. The device of claim 4 in which said digitizer includes means forinhibiting said non-zero-frequency indicating pulse train output inresponse to only a nonzero-frequency component of said sampled videosignal of less than a preselected threshold.

11. In a periodically sampled video signalling system, digital means forgenerating a synthetic video signal in response to a sampled video inputhaving a spectral component within a bandwidth region below the samplingfrequency at which said video input is periodically sampled, andcomprising pulse train generator means responsive to said samplingfrequency for providing a first pulse train;

a first up-down counter responsive to said pulse train for generating adigital output indicative of the integral of the pulse train inputthereto and having a control input;

a differential amplifier responsive to the analog amplitude of each ofsaid sampled video input and said digital output of said first counterfor providing an up-down control signal to said control input of saidfirst counter;

trigger generator means responsive to said differential amplifier forproviding a pulse output in response'to each change of sense of saidup-down control signal; and

coincidence signalling means responsive to the coincidence of the sampleinterval of said sampled video input and a preselected state of saidpulse output of said trigger generator for generating a synthetic videooutput.

12. The device of claim 11 in which said coincidence signalling meanscomprises an output up-down counter having one of an upcount and adown-count input coupled to a source of a reference periodic signal, theother of said inputs being coupled to the output of said triggergenerator means; and

a coincidence gate having a first input coupled to said pulse traingenerator means and a second input coupled to an output of said secondup-down counter.

13. The device of claim 12 in which said downcount input of said outputcounter is coupled to a reference periodic signal source having avariable periodicity.

14. The device of claim 11in which said coincidence signalling meanscomprises an up-down counter having an up-count input responsive to theoutput of said trigger generator means and a down count input responsiveto the output of reference periodic signal source having an adjustableperiodicity; and a coincidence gate having a first input coupled to anoutput of said output counter and a second input responsive to thesampled interval of said sampled video signal for providing a syntheticvideo output in response to the coincidence of a preselected overflowcondition of said second counter and said sampled interval.

15. The device of claim 11 in which said trigger generator includesthreshold means for limiting said trigger output response thereof tosystem sampled video inputs representing a non-zero frequency componentvideo envelope and having at least a preselected threshold amplitude.

16. The device of claim 11 in which said trigger generator means isfurther responsive to said pulse train generator means and includesthreshold means for limiting said pulse output as a response to apreselected number of pulse outputs of said pulse train generatoroccurring between subsequentchanges of sense of said control signal.

17. The device of claim 11 in which said trigger generator comprises asecond up-down counter responsive to said control signal output of saiddifferential amplifier and to said pulse train generator means forindicating the integral of each pulse train associated with a change ofsense of said control signal;

threshold comparator means coupled to said second counter for inhibitingsaid counter in response to an output thereof exceeding a preselectedthreshold; and

monostable signalling means for generating a trigger output in responseto said threshold response to said comparator means.

18. In a periodically sampled video signalling system, digital means forgenerating a synthetic video signal in response to a sampled video inputhaving a spectral component within a bandwidth region below the samplingfrequency at which said video input is periodically sampled, andcomprising pulse train generator means responsive to said samplingfrequency for providing a periodic pulse train;

a first up-down counter responsive to said pulsed train for generating adigital output indicative of the integral of a pulse train input theretoand having an up-down control input;

a differential amplifier responsive to the analog amplitude of each ofsaid sampled video input and said digital output of said first counterfor providing an up-down control signal to said control input of saidfirst counter for closed loop control thereof;

trigger generator means responsive to said frequency generator means andsaid differential amplifier for providing a second pulsed output; and,

coincidence signalling means responsive to the coincidence of saidsampled interval of said sampled video and an output of said triggergenerator for generating a synthetic video output.

19. Digital doppler processing means for utilization in a spacebornemoving target indicating type of pulsed-energy ranging system, andcomprising range gating means responsive to both a video receiver outputand a system trigger of said pulsed energy system and having a pluralityof range-gated outputs, each representing range-gate sampling of asuccessive range bin of a receiver range trace signal;

a like plurality of digital filter means as range gated outputs of saidrange gating means, each said digital filter means comprising adigitizer responsive to a mutually exclusive one of said range gateoutputs for providing a pulse train output indicative of a non-zerofrequency component of the video envelope of the associated range-gatedvideo output, and

coincidence signalling means responsive to the coincidence of the gatinginterval of said range gate output and a preselected state of the outputof said digitizer for generating a synthetic video output; and

signal combining means responsive to said synthetic video outputs ofsaid plurality of digital filter means for providing a doppler-processedAMTI range trace signal.

20. The device of claim 19 in which said coincidance signalling meansincludes bandpass limiting means comprising a voltage controlledoscillator having a periodicity corresponding to a lower limit frequencyof a selected bandpass limit of said filter;

an up-down counter having an up count input coupled to the output ofsaid digitizer and a down count input coupled to an output of saidvoltagecontrolled oscillator; and

a coincidence gate responsive to a preselected output state of saidcounter and the gating interval of said range gate output for generatinga synthetic video output signal.

21. The device of claim 20 in which there is further included a functiongenerator for generating a control voltage indicative of a preselectedfunction for control of said voltage controlled oscillator.

22. The device of claim 20 in which there is further provided means forvarying the frequency of said voltage controlled oscillator inaccordance with the product of the platform velocity and cosine of thelook angle of said pulsed energy ranging system.

23. The device of claim 19 in which said digitizer includes thresholdmeans for limiting the output response thereof to system sampled videoinputs representing a non-zero frequency component video envelope havingat least a preselected threshold amplitude.

1. In a video sampled data system, digital filter means for indicatingthe presence of a spectral component in the video envelope of the outputof said system and above a preselected lower limit frequency, andcomprising: digitizer means responsive to the amplitude and sampledinterval of the sampled video of said system for providing a pulseoutput indicative of a non-zero frequency component of said videoenvelope; periodic pulse generating means having a periodicitycorrespOnding to a selected lower limit bandpass frequency; and anup-down counter having an up count input responsive to the output ofsaid digitizer means and a down count input responsive to the output ofsaid pulse generating means for generating an over flow conditionindicative of the presence of a video envelope spectral component in thefrequency region above said lower limit frequency.
 2. The device ofclaim 1 in which the periodicity of said periodic pulse generating meansis adjustable, whereby said selected lower limit bandpass frequency iscorrespondingly adjusted.
 3. In a video sampled data system, digitalfilter means for indicating the presence of a spectral component in thevideo envelope of the output of said system and above a preselectedlower limit frequency, and comprising: a digitizer means responsive tothe amplitude and sampled interval of the sampled video of said systemfor providing a pulse output indicative of a non-zero frequencycomponent of said video envelope above a preselected thresholdamplitude; periodic pulse generating means having a periodicitycorresponding to a selected lower limit bandpass frequency; and anup-down counter having an up count input responsive to the output ofsaid digitizer means and a down count input responsive to the output ofsaid pulse generating means for generating an overflow conditionindicative of the presence of a video envelope spectral component in thefrequency region above said lower limit frequency.
 4. In a periodicallysampled video signalling system, a bandpass limited digital filterhaving an upper and a lower cut-off frequency and comprising digitizermeans responsive to the sampled video signal and to the samplingperiodicity of said signalling system for providing a pulse trainindicative of a non-zero-frequency spectral content of said sampledvideo signal less than one-half the frequency at which said sampledvideo signal is sampled; and synthetic video means responsive to saidsampling periodicity and to said non-zero-frequency pulse train forproviding an output pulse in synchronism with said sampling periodicity.5. The device of claim 4 in which said synthetic video means includesmeans for limiting the lower cut-off frequency of the bandpass of saiddigital filter and comprising a reference periodic pulse generatorhaving a frequency less than one-half the sampling frequency of saidsampled signalling system and corresponding to a lower limit frequencyof the bandpass of said filter; an up-down counter having an up-countand a down-count input, one of said inputs being responsive to a pulsetrain output of said digitizer and a second input responsive to a pulsetrain output of said reference pulse generator; and coincidencesignalling means responsive to said sampling periodicity and apreselected condition of said counter for generating an output signal.6. The device of claim 5 in which the frequency of said referenceperiodic pulse generator is adjustable, whereby said lower cut-offfrequency of said digital filter may be correspondingly adjusted.
 7. Thedevice of claim 6 in which there is further included means for varyingthe frequency of said reference periodic pulse generator as a functionof the velocity of a utilizing vehicle.
 8. The device of claim 6 inwhich there is further included means for varying the frequency of saidreference periodic pulse generator as a function of the look angle of autilizing sensor system.
 9. The device of claim 6 in which there isfurther included means for varying the frequency of said referenceperiodic pulse generator in accordance with the product of vehiclevelocity and cosine of sensor look angle of a utilizing sensor mountedon a moving platform.
 10. The device of claim 4 in which said digitizerincludes means for inhibiting said non-zero-frequency indicating pulsetrain output in response to only a non-zero-frequency component of saidsampled video signal of less than a preselected threshold.
 11. In aperiodically sampled video signalling system, digital means forgenerating a synthetic video signal in response to a sampled video inputhaving a spectral component within a bandwidth region below the samplingfrequency at which said video input is periodically sampled, andcomprising pulse train generator means responsive to said samplingfrequency for providing a first pulse train; a first up-down counterresponsive to said pulse train for generating a digital outputindicative of the integral of the pulse train input thereto and having acontrol input; a differential amplifier responsive to the analogamplitude of each of said sampled video input and said digital output ofsaid first counter for providing an up-down control signal to saidcontrol input of said first counter; trigger generator means responsiveto said differential amplifier for providing a pulse output in responseto each change of sense of said up-down control signal; and coincidencesignalling means responsive to the coincidence of the sample interval ofsaid sampled video input and a preselected state of said pulse output ofsaid trigger generator for generating a synthetic video output.
 12. Thedevice of claim 11 in which said coincidence signalling means comprisesan output up-down counter having one of an up-count and a down-countinput coupled to a source of a reference periodic signal, the other ofsaid inputs being coupled to the output of said trigger generator means;and a coincidence gate having a first input coupled to said pulse traingenerator means and a second input coupled to an output of said secondup-down counter.
 13. The device of claim 12 in which said downcountinput of said output counter is coupled to a reference periodic signalsource having a variable periodicity.
 14. The device of claim 11 inwhich said coincidence signalling means comprises an up-down counterhaving an up-count input responsive to the output of said triggergenerator means and a down count input responsive to the output ofreference periodic signal source having an adjustable periodicity; and acoincidence gate having a first input coupled to an output of saidoutput counter and a second input responsive to the sampled interval ofsaid sampled video signal for providing a synthetic video output inresponse to the coincidence of a preselected overflow condition of saidsecond counter and said sampled interval.
 15. The device of claim 11 inwhich said trigger generator includes threshold means for limiting saidtrigger output response thereof to system sampled video inputsrepresenting a non-zero frequency component video envelope and having atleast a preselected threshold amplitude.
 16. The device of claim 11 inwhich said trigger generator means is further responsive to said pulsetrain generator means and includes threshold means for limiting saidpulse output as a response to a preselected number of pulse outputs ofsaid pulse train generator occurring between subsequent changes of senseof said control signal.
 17. The device of claim 11 in which said triggergenerator comprises a second up-down counter responsive to said controlsignal output of said differential amplifier and to said pulse traingenerator means for indicating the integral of each pulse trainassociated with a change of sense of said control signal; thresholdcomparator means coupled to said second counter for inhibiting saidcounter in response to an output thereof exceeding a preselectedthreshold; and monostable signalling means for generating a triggeroutput in response to said threshold response to said comparator means.18. In a periodically sampled video signalling system, digital means forgenerating a synthetic video signal in response to a sampled video inputhaving a spectral component within a bandwidth region below the samplingfrequency at which said video input is periodically sampled, andcompRising pulse train generator means responsive to said samplingfrequency for providing a periodic pulse train; a first up-down counterresponsive to said pulsed train for generating a digital outputindicative of the integral of a pulse train input thereto and having anup-down control input; a differential amplifier responsive to the analogamplitude of each of said sampled video input and said digital output ofsaid first counter for providing an up-down control signal to saidcontrol input of said first counter for closed loop control thereof;trigger generator means responsive to said frequency generator means andsaid differential amplifier for providing a second pulsed output; and,coincidence signalling means responsive to the coincidence of saidsampled interval of said sampled video and an output of said triggergenerator for generating a synthetic video output.
 19. Digital dopplerprocessing means for utilization in a spaceborne moving targetindicating type of pulsed-energy ranging system, and comprising rangegating means responsive to both a video receiver output and a systemtrigger of said pulsed energy system and having a plurality ofrange-gated outputs, each representing range-gate sampling of asuccessive range bin of a receiver range trace signal; a like pluralityof digital filter means as range gated outputs of said range gatingmeans, each said digital filter means comprising a digitizer responsiveto a mutually exclusive one of said range gate outputs for providing apulse train output indicative of a non-zero frequency component of thevideo envelope of the associated range-gated video output, andcoincidence signalling means responsive to the coincidence of the gatinginterval of said range gate output and a preselected state of the outputof said digitizer for generating a synthetic video output; and signalcombining means responsive to said synthetic video outputs of saidplurality of digital filter means for providing a doppler-processed AMTIrange trace signal.
 20. The device of claim 19 in which said coincidancesignalling means includes bandpass limiting means comprising a voltagecontrolled oscillator having a periodicity corresponding to a lowerlimit frequency of a selected bandpass limit of said filter; an up-downcounter having an up count input coupled to the output of said digitizerand a down count input coupled to an output of said voltage-controlledoscillator; and a coincidence gate responsive to a preselected outputstate of said counter and the gating interval of said range gate outputfor generating a synthetic video output signal.
 21. The device of claim20 in which there is further included a function generator forgenerating a control voltage indicative of a preselected function forcontrol of said voltage controlled oscillator.
 22. The device of claim20 in which there is further provided means for varying the frequency ofsaid voltage controlled oscillator in accordance with the product of theplatform velocity and cosine of the look angle of said pulsed energyranging system.
 23. The device of claim 19 in which said digitizerincludes threshold means for limiting the output response thereof tosystem sampled video inputs representing a non-zero frequency componentvideo envelope having at least a preselected threshold amplitude.